Question: $\sum\limits_{j=1}^{3 }{{(3j)}}=$
Explanation: What is the question asking for? $\sum$ tells us to find the sum. The question is asking for the sum of the values of $3j$ from $j = 1$ to $j = 3$. Evaluating $\begin{aligned} \sum\limits_{j=1}^{3 }{({3j})}&= (3(1)) + (3(2)) + (3(3)) \\\\ &= 3 + 6 + 9 \\\\ &= 18\end{aligned}$ The answer $\sum\limits_{j=1}^{3 }{({3j})}=18$